Herbs, shrubs [or trees], annual or perennial. Roots fibrous to fleshy or tuberous. Stems pro-cumbent to erect, sometimes clambering through other plants. Leaves opposite [rarely alternate], exstipulate, sessile or petiolate, pairs equal or unequal; blade linear to ovate or ± round, thin to thick and fleshy or succulent, plane or undulate, base ± symmetric to strongly unequal, margins entire or sinuate, glabrous or pubescent. Inflorescences terminal or lateral in leaf axils, usually cymose or racemose, open or congested, bracteate; bracts distinct or connate, sometimes forming an involucre containing 1-80 flowers, when containing only 1 flower, calyxlike, sometimes brightly colored. Flowers unisexual or bisexual, sometimes cleistogamous, incomplete, showy or inconspicuous; calyx usually 5 connate sepals, varying from small and not showy to large, notably petaloid, and colorful, corollalike; corolla absent; stamens commonly as many as calyx lobes and alternate with them, varying 1-18[-30]; filaments connate basally [or distinct]; gynoecium 1-carpellate; styles filiform; stigmas capitate, fusiform to linear, or ending in tufts of hairs; ovule 1; placentation basal. Fruits aerial (hypogeous in Okenia), accessory, radially symmetric or dorsiventrally compressed anthocarps, consisting of achene or utricle enclosed in fleshy, leathery, or woody base of calyx, sides longitudinally 5- or 10-ribbed or not ribbed, glandular or not, smooth, wrinkled, or warty, glabrous or pubescent; ribs, when present, varying from fine and linear to broad and obtuse ridges, or thick to thin wings, or rows of curved gland-tipped teeth. Seed 1; seed coat membranaceous; embryo straight or curved; cotyledons foliaceous; endosperm scanty; perisperm abundant. x = 9 or 10- Fruits of Acleisanthes, Allionia, most Anulocaulis, Boerhavia, Cyphomeris, and many of Mirabilis produce mucilage when wetted. Bougainvillea glabra Choisy, a native of South America, is a woody, scandent plant grown on walls and trellises, with brilliant red-purple, orange, yellowish, or white bracts providing spectacular color. It may persist about old dwellings, and occasionally in plant dumps and gardens in warmer parts of the flora area, but it has apparently not naturalized.

Classification at the generic and suprageneric levels is problematic in Nyctaginaceae. Two great students of the family recognized differing numbers of tribes: A. Heimerl (1934c) five worldwide, and P. C. Standley (1918) six in North America alone. J. Hutchinson (1959) recognized eight tribes worldwide. Many genera are small, and G. H. M. Lawrence (1951) noted that 50% of the genera are monotypic. P. C. Standley, particularly in his earlier works (1911, 1918), recognized segregate genera, especially from widespread and variable genera such as Mirabilis and Pisonia as those are treated here. Whereas there has been a tendency among recent authors to accept a widely inclusive genus concept for Mirabilis (A. L. Bogle 1974), the segregate genera from an inclusive Boerhavia have been mostly retained (e.g., C. F. Reed 1969), although this view has not been completely accepted (F. R. Fosberg 1978).

Nyctaginaceae is believed to have been derived from Phytolaccaceae or a close common ancestor (A. Cronquist 1981), a relationship supported by J. Rodman et al. (1984) and S. R. Downie and J. Palmer (1994). Considerable anatomical work has increased our understanding of unique structural attributes in the family, and also often of its position within the Caryophyllales (e.g., H.-D. Behnke 1972, sieve-tube plastids; E. Balfour 1965, anomalous nature of secondary wood; J. W. Nowicke 1970 and J. W. Nowicke and T. J. Luikart 1971, pollen morphology; A. D. Joshi and V. S. Rao 1934, vascular anatomy of perianth).

As A. L. Bogle (1974) discussed, the term 'anthocarp,' traditionally applied to the accessory fruit, has been defined in several ways and is an ambiguous term. Technically the fruit is a diclesium, as defined by A. E. Radford et al. (1974). This term has not been applied in descriptions of taxa in the family, and for these reasons the general term 'fruit,' as used in the descriptions of taxa that follow, refers to the accessory fruit of this family. The side walls of the diclesium may bear sticky stipitate glands that are involved in dispersal by animals. The surfaces